Diameter

In geometry, a diameter of a circle is a straight line segment that goes through the center of the circle and has its ends on the circle. It is also the longest chord of the circle. This idea can also be used for the diameter of a sphere.

Today, when we talk about the diameter, we often mean the length of this line. For any circle or sphere, all diameters have the same length. This length is always twice the length of the radius, which is the distance from the center of the circle to any point on its edge.

The word "diameter" comes from Ancient Greek. It is sometimes shortened to DIA, dia, d, or ∅.

Constructions

You can make a diameter of a circle using a straightedge and compass. First, pick any line across the circle, called a chord. Then, draw a line that cuts this chord exactly in half and goes straight through the middle of the circle. If you draw two of these lines, where they cross is the center of the circle.

To make a circle with a specific line as its diameter, find the middle point of that line. Then, draw the circle so it centers on that middle point and goes through one end of the line.

Symbol

"⌀" redirects here. For other uses, see ⌀ (disambiguation).

The symbol ⌀ is used in technical drawings and specifications to show the diameter of something. For example, you might see "⌀ 55 mm" which means the diameter is 55 millimeters. This symbol is also used for sizes of camera filters.

The symbol has a special code in Unicode called U+2300 ⌀ DIAMETER SIGN, and it is part of the Miscellaneous Technical set. It looks similar to some other symbols, like Ø or ∅, but these have different meanings.

Generalizations

The idea of a diameter can be used for many shapes beyond just circles and spheres. For any group of points, the diameter is the greatest distance between any two points in that group.

For shapes like ellipses, a diameter is any line that goes through the center of the shape. The longest diameter of an ellipse is called the major axis, and the shortest is called the minor axis. Some special diameters are called conjugate diameters.

We can also find an equivalent diameter for objects that are not perfect circles or spheres. This is the diameter of a circle or sphere that would have the same effect as the object. For example, the hydraulic diameter measures the effective size of a pipe for fluid flow.

The diameter of a circle is always twice its radius, but this special rule only works for circles in flat geometry. Other shapes have different relationships between their size and their diameters.